1. Field of the Invention
The invention relates to a drill head as used in geological exploration, particularly adapted for, but not limited to, use as a core sampling drill head such as used in diamond drilling.
2. Prior Art
Two main types of drill heads are known to the present inventor and in both types the drill head has a powered drill chuck having chuck jaws movable inwards to grip a drill rod to rotate the rod about a drill rod axis and to resist axial forces on the rod. In the first type the jaws are extended or closed by a hydraulically applied force, and in the second type the jaws are extended by force from mechanical springs. The first type is satisfactory for many applications, but when drilling upwards a hydraulic pressure failure would probably result in the jaws losing grip on the drill rod. If grip of the drill rod is lost the string of drill rods in the hole is likely to fall from the hole resulting in possible injury to operators and damage to the drilling apparatus. In the second type of drill head, closure of the jaws is essentially failsafe with regard to hydraulic pressure failure, but size of the springs necessary to provide sufficient grip for a practical range of diameters of drill rods results in a drill head of impractical length for use underground.
There is an inherent problem in using compression springs to actuate jaws of a drill chuck. It is well known that force from the compression springs decreases with increase in the extension of the springs. Thus, when a worn rod, that is a rod of reduced diameter, is gripped by the jaws, or when the chuck itself is worn, commonly there is insufficient gripping force available due to excessive extension of the chuck springs, and thus slippage between the jaws and rod is likely. Range of accommodation of the springs can be increased by increasing the length of the springs, but this increases the axial length of the drill head, thus restricting use of the drill in confined spaces. Furthermore, other tools, e.g. drill reaming shells, are sometimes required for passing through the drill head, which tools commonly have a larger diameter than the drill rods. Such tools would interfere with the chuck jaws if the chuck jaws cannot retract further than required for drill rods so as to leave sufficient space to pass the tools. Retraction of the jaws to pass larger diameter tools requires overcoming excessive chuck spring force, which force increases with retraction of the springs, or with increase in length of springs. Also, with many conventional drill heads, replacement or servicing of the chuck and associated components requires extensive dismantling of the machine. Also, in some drill chucks, reaction from forces maintaining the jaws gripping the rod is carried by bearing journalling the chuck, which bearings thus have to be correspondingly larger than otherwise required for resisting drilling forces alone.